In general, a display device used as a computer monitor, a television (TV), or the like includes a liquid crystal display (LCD). The LCD is not capable of emitting light and thus requires a separate light source.
As a light source for an LCD, a plurality of fluorescent lamps such as cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs), and so on or a plurality of light emitting diodes (LEDs) are used. Such a light source is installed in a backlight unit (BLU) together with a light guide plate, a plurality of optical sheets, a reflective plate, and so on.
Recently, an LED among these light sources has attracted attention as a next generation light source due to low power consumption, high durability, and low manufacturing costs.
However, when an LED is used as a light source, light tends to be intensively emitted to a narrow region, and thus there is a need to uniformly distribute light to a wide region in order to apply the LED to a surface light source such as a display device.
Accordingly, recently, research has been actively conducted into a lens for an LED, for performing this function. Among these technologies, representative prior arts are disclosed in Korean Patent Registration No. 10-0971639, Korean Patent Registration No. 10-0977336, and so on.
A lens for an LED according to the prior arts has a continuously curved surface as a top surface for uniformly distributing light emitted from the LED. In particular, the top surface of the lens is a curved surface that is convex away from an optical axis of the LED, thereby increasing an overall volume of the lens. Accordingly, there is a problem in that material costs are increased in that a general lens is manufactured via injection molding, and there is a problem in that one injection molding cycle is lengthened to increase manufacturing time.
Due to reflection that occurs while light passes through a boundary surface between materials with different refractive indexes of light, that is, Fresnel reflection, a portion of light emitted through an emitting surface of a lens is reflected to a bottom surface of the lens, and the reflected light is re-reflected by the bottom surface of the lens or a reflective sheet disposed below the lens and is emitted from the emitting surface close to an optical axis of an LED. Accordingly, the lens according to the prior arts has a problem in terms of brightness variation in the vicinity of the optical axis.
The problem in terms of brightness variation due to Fresnel reflection is disclosed in comparatively detail in page 11 and FIGS. 13 to 16 of Korean Patent Registration No. 10-0977336 (hereinafter, referred to as ‘prior art’) among the prior arts. In order to overcome this problem, the prior art discloses a lens configured in such a way that a light scattering portion is disposed at a position corresponding to a light concentrating portion of a bottom surface, on which light reflected due to Fresnel reflection is concentrated.
However, as disclosed in an upper portion of page 12 and FIG. 17 of the prior art, when the light scattering portion is configured above, there is a problem in that incident light is not uniformly distribute directly to the light scattering portion from an LED, and thus in order to overcome this problem, a separate light scattering surface needs to be further configured like in an embodiment illustrated in FIG. 18 of the prior art.